Hydraulic fracturing, also known as fracking, is a known process which allows creation of fractures from a wellbore into a formation or reservoir. This technique consists of pumping hydraulic fluid into a wellbore at a pressure and injection rate such that fractures are created into the formation. These fractures act as channels which facilitate and increase production of hydrocarbons, e.g. oil, from the formation into the wellbore.
In order to prevent closure or collapse of these fractures under reservoir conditions when the hydraulic fracture pressure is released, it is known to inject solid particles known as proppants into the created fractures, either during, or after, fracturing. The proppants act to “prop” open the fractures once the hydraulic fracturing has ceased.
A number of materials can be used as proppants, including sand particles such as natural sand particles and/or resin-coated sand particles, ceramic beads, glass beads, and the like.
A problem with the use of proppants is that, during subsequent production of the formation, flow of hydrocarbons and/or of aqueous fluids tends to cause a significant amount of proppant to be produced back into the wellbore, potentially damaging equipment, and requiring separation from the produced hydrocarbons.
Various attempts have been made to seek to consolidate proppants in the formed fractures. Such attempts have been disclosed for example in GB 1,327,735 (Texaco Development Corp), U.S. Pat. No. 6,582,819 (Borden Chemical Inc), U.S. Pat. No. 7,044,224 (Halliburton Energy Services Inc), which disclose the use of a cement composition to cement the proppants in place. Other attempts have been disclosed for example in US 2012/0261126 Halliburton Energy Services Inc. U.S. Pat. No. 4,785,884 (Acme Resin Corp.), U.S. Pat. No. 5,604,184 (Texaco Inc.), U.S. Pat. No. 5,924,488 (Halliburton Energy Services Inc), U.S. Pat. No. 6,705,400 (Halliburton Energy Services Inc), U.S. Pat. No. 6,877,560 (Halliburton Energy Services Inc.), U.S. Pat. No. 7,963,330 (Halliburton Energy Services Inc.), U.S. Pat. No. 8,136,595 (Halliburton Energy Services Inc.), WO 2009/078745 (Schlumberger Holdings Limited), and WO 2009/088315 (Schlumberger Holdings Limited), which disclose the use of a polymeric resin to bind the proppants in place. However, a problem with the above prior art methods is that the binder material, whether a cement or a polymeric resin, significantly reduces the permeability of the fractures, thus reducing production rates. Another problem is that, if pumped wrongly, such binder materials cannot be removed from the well and may damage the well permanently.
It is also known to use enzymes systems, whereby calcium carbonate is precipitated in situ, in methods of sealing or plugging subterranean formations, as disclosed for example in WO 2008/119620 (Maersk), WO 2005/124099 (Statoil), US 2012/0269584 (Temasi AS), U.S. Pat. No. 7,841,804 (Impermeable AS), WO 2009/008724 (Stichting Deltares), U.S. Pat. No. 4,232,740 (Texaco Development Corp), U.S. Pat. No. 5,088,555 (Mobil Oil Corp), U.S. Pat. No. 5,101,901 (Mobil Oil Corp.), U.S. Pat. No. 7,975,764 (Schlumberger Technology Corp.), U.S. Pat. No. 8,124,571 (Cleansorb Limited). However, these documents only disclose using such enzyme systems in order to reduce water production by reducing permeability of the formation, or to consolidate the formation itself.